Antenna array has been widely used in base stations. It generally comprises a set of individual antenna elements. In combination with MIMO technologies, the capacity of wireless channel may be increased without increasing the bandwidth and transmission power. Recently, massive MIMO (also known as large-scale antenna systems, very large MIMO, or hyper MIMO) has been developed. It makes use of a very large amount (e.g., hundreds or thousands) of service antennas that are operated fully coherently and adaptively. Thereby, the transmission and reception of signal energy can be focused into ever-smaller regions of space. This can bring huge improvements in throughput and energy efficiency, in particular when combined with simultaneous scheduling of a large number (e.g., tens or hundreds) of user terminals. Other benefits of massive MIMO include the extensive use of inexpensive low-power components, reduced latency, simplification of the media access control (MAC) layer, and robustness to interference and intentional jamming.
Beamforming is a signal processing technique usually used in antenna array of MIMO system for directional signal transmission or reception. This may be achieved by combining antenna elements in a phased array in such a way that signals at particular angles experience constructive interference while others experience destructive interference. Beamforming can be used at both the transmitting and receiving ends in order to achieve spatial selectivity.
Analog beamforming implementations usually rely on a list of predefined beams (or codebooks) that can be selected to transmit/receive data streams. Beams can be set to form sectors, hotspots, or some spatial separations to allow user multiplexing. An antenna array spanning over two dimensions can perform both vertical and horizontal beam shaping. It is generally desirable to have a set of beams that can span the entire desired coverage area of the antenna array to avoid coverage holes for example.
In receiver side, analog beamforming is a method that applies weights (usually phase shifts) to the signals from the antenna elements and combines them in order to increase the antenna gain from a given direction. This combining is done in an analog domain, i.e. before the signal is converted to a digital stream. The result of such combining is treated as a digital stream by a digital front-end and a baseband processing unit.
FIG. 1 is a schematic architectural diagram showing a receiver having an analog beamforming combiner. As shown, the receiver 100 may comprise a plurality of antenna elements 102 for receiving radio frequency (RF) signals. Here, four antenna elements are shown as an exemplary example. The RF signals may be processed by respective RF processing units 104 such that the processed signals are provided to a beamforming combiner 106. The beamforming combiner 106 may apply phase shifting to the processed signals by a plurality of phase shifters 108, and combine the signals after phase shifting, wherein the coefficients of the phase shifters are beamforming weights. The combined signal may be converted to a digital signal by an analog-to-digital converter (ADC) 110, and may be further processed by a digital front-end 112.
The architecture shown in FIG. 1 is usually used in beam sweeping. FIG. 2 is a schematic diagram of beam sweeping. As shown, beam sweeping is a method used to scan the desired coverage area with beams in corresponding directions. This is needed when the location of a user is not known correctly, such as during random access procedure. To make sure that the desired coverage area is well covered, the transmitter tries all the beam directions available. For example, three beams with different beam indices (e.g., beam index 1, beam index 2 and beam index 3) are shown in FIG. 2 for illustration. To achieve maximum coverage, the beams used for covering respective directions are usually configured with a maximum gain.
During beam sweeping, the power levels and phases of signals received from the antenna array may change rapidly, and thus receiving beam control is performed in the receiver. However, when existing solutions for receiving beam control are used in a massive MIMO, new problems arise, resulting in the performance degradation or very high cost of the receiver. Therefore, it would be desirable to provide an effective and low cost solution for receiving beam control.